1. Field of the Invention
This invention relates to a position detecting device for use in, for example, a machine tool for metal machining, an industrial machine, a robot or the like.
2. Description of the Related Art
In a main body apparatus for various machine tools, industrial robots or the like, a position detecting device for detecting the shift amount and shift position of a movable portion such as a table or the like is installed. In recent years, there are many cases in which an encoder scale of a so-called absolute type is used as the position detecting device in the industrial machines as a whole typically including the machine tools.
The encoder scale of an absolute type has such a structure that by reading non-repetitive codes recorded on the scale, the amount of mechanical displacement, as it is, is outputted as absolute positional information by using binary codes or the like. Since the encoder scale of an absolute type makes it possible to always confirm the position relative to the origin, it is not necessary to detect the origin (return to the origin) every time the power supply is turned on. Moreover, even when the detection head deviates from the scale, by returning the detection head to a predetermined position on the scale, the absolute position of the corresponding point can be obtained at once.
In the case of a linear encoder of an incremental type that has been often used conventionally, upon activating the device or upon occurrence of a trouble, it is necessary to carry out the returning process to the origin; however, the encoder scale of an absolute type is advantageous in that no returning process to the origin is required.
As the encoder scale of an absolute type, a magnetic absolute-type encoder, which generates an absolute-value-forming code by using a non-repetitive pattern (M-code pattern of an M-sequence or the like) formed by combining polarized portions and non-polarized portions of the magnetic scale with one another so that absolute data are outputted, has been known (for example, see Patent Documents 1 and 2, Japanese Patent Application Laid-Open No. 9-264760, Japanese Patent Application Laid-Open No. 2007-033245).
In the magnetic absolute-type encoder, the absolute-value-forming code is read from the magnetic scale in which the pattern codes of the absolute-value-forming code using the M-code pattern are recorded, by using magneto-resistive effect elements (MR elements).
As the magneto-resistive effect elements (MR elements), AMR elements using the Anisotropic Magneto-Resistive (AMR) effect of a ferromagnetic metal (for example, Ni—Fe film having a small saturated magnetic field, Ni—Co film having a great MR rate of change, or the like), magneto-resistive effect elements (GMR elements) using a coupling-type Giant Magneto-Resistive (GMR) effect by the use of a magnetic film composed of a multi-layer structure, etc. have been known.
In the AMR elements, an electric resistance is allowed to change by a comparatively low change in magnetic field (several Oe to several tens f Oe).
Moreover, the GMR elements have a greater rate of change in resistance in comparison with the AMR elements, and consequently make it possible to provide a better spacing characteristic and a higher output in comparison with the AMR elements. Moreover, in the magnetic film that exerts the GMR effect, since the resistance change is exerted isotropically regardless of a relative angle between the magnetic field and the electric current, by disposing a signal magnetic field from a detection subject material and the longitudinal direction of a field sensitive pattern in parallel with each other, the influences from a diamagnetic field can be reduced.
Moreover, in the case of a position detecting device in which an incremental (INC) track having S poles and N poles that are alternately placed side by side regularly and an absolute (ABS) track having absolute-value forming codes using the M-code pattern recorded thereon are used in combination, as shown in FIG. 34, [1] and [0] of the M-code to be recorded in the ABS track corresponds to 1 pitch of the INC track=1 wavelength (λ) of a reproducing signal, and in response to information of codes, “presence of record” and “absence of record” are recorded in a medium for each of INC 1 wavelength (λ).
In this case, the AMR elements can measure the absolute value of a magnetic field; however, they cannot distinguish the polarity.
That is, in the case of a detection using the AMR elements, the MR elements basically use a characteristic in which a resistance value is changed depending on the size of a magnetic field applied to an element in a direction orthogonal to the longitudinal direction of the element stripes. At this time, as long as a magnetic field is applied, regardless of the direction of the magnetic field, the resistance value is reduced from the value at the time of no magnetic field. Therefore, this method fails to distinguish the polarized direction.
In the case when the AMR elements are used, pieces of information of [1] and [0] or [H] and [L] in the respective bits of the M-code correspond to “presence of record” and “absence of record” as pieces of magnetic information.
However, when “presence of record” and “absence of record” are recorded in the medium in association with information of [1] and [0] of the M-code for each of INC 1 wavelength, the bit of “presence of record” adjacent to a bit corresponding to “absence of record” has an expanded recording width, with the result that it becomes difficult to record/reproduce a signal correctly corresponding 1 pitch of the INC track=1 wavelength of reproducing signal.
For this reason, in the case when the bit of “presence of record” adjacent to a bit corresponding to “absence of record” is formed (recorded), recording has to be carried out so as to reproduce information as correctly as possible by optimizing the recording conditions; however, such optimizing processes are not simple and also influenced by the order of sequence of codes.
For example, the conditions for optimizing are different depending on a case in which there are bits of “presence of record” on two sides, with “absence of record” being located therebetween, and a case in which there are continuous bits of “presence of record” and the bit of “absence of record” continues from the adjacent side.
In this manner, the optimizing processes need to be carried out in accordance with the positions of codes, causing very complicated optimizing processes.
Moreover, in the case when information is read out from the recording medium formed as described above, the size of a magnetic field from the medium is reduced as the distance from an MR element to the medium becomes longer; however, since the optimized recording fails to have a uniform magnetization intensity, a uniform reduction is not prepared relative to a change in distance, with the result that the reproduced waveform tends to change. Consequently, the distance range for an effective reproducing process becomes narrow and is limited.
On the other hand, in order to avoid the above-mentioned problem, a method has been proposed in which a bias magnetic field is applied in a direction orthogonal to the longitudinal direction of the MR elements so that by moving the diamagnetic field operation point to the mid point of a change, the direction of a magnetic field is distinguished (hereinafter, referred to as an operation point bias).
In this case, it is necessary to apply a uniform bias laterally onto a group of sensors for use in detecting M-codes, and since a method by the use of a bias magnet causes a defect in that a large magnet is required, resulting defects are a large-size device and high costs from the necessity of providing a long distance from the INC track located on the side thereof, or the like.
Moreover, in the case when another magnetic device other than the MR element, for example, a hole element, is used, although the above-mentioned bias is not required, it is difficult to configure a system capable of obtaining a resolution of 10 nm to sub-microns required for tool machines and industrial machines in the case of a configuration using a general-use hole element.
Furthermore, in the following description that can be applied to the magnetic type devices as a whole, in positions where the same pieces of information of [1] and [0] or [H] and [L] in the respective bits of M-codes continue successively, recording processes in the same direction, that is, polarizing processes, are carried out as many as the number of the continuous positions, with the result that the polarized region having the same continuous information is brought into a state equivalent to a structure in which a long magnet is formed. For this reason, depending on relationships between the continuous length and the spacing distance of the MR elements, as indicated by a solid line in FIG. 34, in the vicinity of the center in the longitudinal direction of the polarized region having the same continuous information, that is, the long magnet, a magnetic field applied to the MR element becomes weaker, causing a reduction in the signal and sometimes resulting in a detection error.
In the same manner, in the following description that can be applied to the magnetic type devices as a whole, in general, in the magnetic type, with respect to a series of recording pitches, since, as the spacing distance becomes larger, the intensity of the resulting magnetic field is reduced virtually exponentially in accordance with the principle of magnetism, the amount of change in the element fluctuates greatly relative to fluctuations in the spacing. For this reason, the application region is limited. Moreover, it is difficult to form a stable signal in the application region.
In view of the above-mentioned problems, the magnetic absolute scale encoder that is commercialized at present adopts AMR elements for use as machine tools and industrial machines requiring a resolution of 10 nano-level, with no operation-point bias being used.
Therefore, the binary information is dependent on the presence or absence of magnetic recording.
Moreover, on the scale side, drums and disc-shaped members having a length exceeding 4 m or various diameters are required, and from the viewpoint of costs, media having a possibility of practical use are those coated media, those alloy magnet media or the like, and any of these have an anisotropic property or an isotropic property in the longitudinal direction. No vertical media that can achieve these shapes and precision have been realized in practical use.
On the other hand, in the case of industrial apparatuses for use in transporting purposes, or the like, that do not require so much resolution and precision as described above, a rubber magnet or the like may be used as a medium even when the same MR elements are used so that a vertical magnetic medium may be practically used.
Moreover, in this system, since the bit corresponding to “presence of record” is the same regardless of polarized directions, the continuous bits can be alternately polarized. Thus, the problem in that the continuous bits cause one big magnetic pole can be solved.
At this time, in the vicinity of a position where polarized directions are changed, the magnetic field becomes weaker to form the same state as that of “absence of record” so that the resulting signal is reversed; however, as shown in FIG. 34, more ABS sensors than the number of required bits are disposed so as to correct deviations between ABS bits as well as between INC and ABS; thus, by providing such a configuration as to prevent deviations in scale as they are from forming a read error signal, the problem can be prevented.
In this method, an area for use in actually detecting the ABS corresponds to one portion in a range from 50% to 60% or the like of 1 bit length (derived from setting and designing of the detection system and a degree of additional margin) so that a stable region of a signal can be detected.
In other words, the detection output of the ABS sensor positioned in the vicinity of the position where polarized directions are changed, which is indicated by half-tone dot meshing in FIG. 36, is not utilized.
In this configuration, since the ABS track portion in the magnetic scale is dependent on the presence or absence of magnetic recording of binary information, the bit of “presence of record” adjacent to a bit corresponding to “absence of record” as described above has an expanded recording width, with the result that it becomes difficult to record/reproduce a signal correctly corresponding to 1 pitch of the INC track=1 wavelength of reproducing signal. For this reason, many problems are raised in that, for example, upon forming (recording) a bit of “presence of record” adjacent to the bit corresponding to “absence of record”, recording has to be carried out so as to reproduce information as correctly as possible by optimizing recording conditions.
In the case when the operation point bias is used, a binary signal is obtained by controlling polarized directions. In this case, no non-polarized portion is located so that in comparison with the presence of non-polarized portion, it is advantageous in that a recording state and recording conditions for obtaining a more correct signal can be easily prepared. However, this configuration, as it is, causes the same polarized direction in the continuous units so that the aforementioned problems remain.
As a method for solving this problem, a system referred to as a frequency modulating system, a Bi-Phase-Space system or the like has been known as one method of digital magnetic recording.
In this system, even in the case of the same-value continuous bits, the above-mentioned problems do not occur because of flux reversal.
However, in this system, the same-value continuous bits double the recording frequency (half of the wavelength).
Therefore, the magnetic field intensity to be read by the detection sensor is reduced and the spacing characteristic deteriorates. Fluctuations in magnetic field relative to spacing become greater to cause a great signal change. Moreover, the application of this system to a scale causes a relationship in which just a double frequency is formed, and this state, as it is, causes the effective detection range in one bit to be limited to 50%, and in this case also, since the binary state is distinguished by, so to speak, the positive or negative of the signal, a problem further arises in that the effective detection range might become 50% or less in the case of occurrence of fluctuations.
The present invention has been devised in view of these circumstances, and its object is to provide a position detecting device capable of obtaining a stable recording state without using “absence of record” upon reproducing a binary signal.
Other objects of the present invention and specific advantages obtained by the present invention will become apparent from the following detailed description of the preferred embodiments of the invention.